Data path timing for data from external ram to LCD

[amirke]

Hi,

I want to know what is the limiting parameter in the process of generating images on an LCD.

Data is generated in an external SDRAM 100Mbps by the controller (for example: NXP LPC1788 / 1850 / 4350 or Freescale kinetis k70)
Then Data need to be taken from SDRAM (through DMA? ) to LCD controller databus.
Then LCD controller activates LCD (lets say 800X600) with the right data.

Is this path is correct ?
What is the timing limits for a maximum data trasnfer to the LCD ?

 

[D.A.(Tony)Stewart]

800x600 x3B x60Hz = 86.4 MB/s or 691.2Mb/s
Some use Serial Rx/Tx such as sn75lvdt1422. 175 Mbps
Others go faster >1Gbps
https://www.ti.com/lit/ds/symlink/sn75lvdt1422.pdf

 

[amirke]

OK.

This is amount of data in a second.
Why 60Hz ?
Isn't 30Hz or 30FPS is enough to make a continuous image on LCD ?

What is the SERIALIZER/DESERIALIZER for ?
For transferring data from microcontroller to LCD ? I want to use an LCD controller which is embedded on some controllers such as NXP LPC1788 / 1850 / 4350 or Freescale kinetis k70.
Working with built in LCD controller is easier , isn't it ?

 

[amirke]

What do you prefer : internal or external LCD controller ?

 

[D.A.(Tony)Stewart]

I prefer analog or DAC direct mux'ing to display. But this is more complex. The limiting factor is usually the input bandwidth of the video unless it is a low resolution display< 2Mpix

I like the K70.

 

[amirke]

I like the K70 as well.

But it cant achieve the data rate you calculated before : " 86.4 MB/s or 691.2Mb/s"
The FlexBus is 50MHZ max.
SPI is 10Mbps maximum.

So my conclusion is that the K70 isn't suitable for my application, correct ?

And can you explain me, why 60Hz ? and what is the differences from 30FPS ?

thanks,
Amirke

 

[D.A.(Tony)Stewart]

I believe the K70 supports 800x600 with high speed DAC so its parallel data in many formats. but 16 bit rgb

 

[amirke]

I don't understand

Do you mean with internal or external LCD controller ?
Cause with external controller, the data rate is not enough.
Can you explain ?

And can you explain what is the difference between 60Hz refresh rate and 30FPS ?

Thanks,
Amir

 

[D.A.(Tony)Stewart]

K70 interfaces all the signals needed for the LCD.

You can update data at any rate you want, but the LCD must be refreshed using 50 or 60 Hz to prevent decay of charges in cells/

Remember that LCDs are like an analog matrix of memory charges that contain a voltage to indicate how much brightness of light is allowed to transmit thru ( on transmissive TFT's) or reflect ( on reflection types) using a diffused light source.
THe K70 will take data and convert it analog with DAC and send to display as an analog voltage. If the scanning stopped, there is a risk that the charge would become trapped or decay to a different value and display as distortion, so refresh rates are standardized. But you dont have to keep sending data from RAM, I believe.

the

 

[D.A.(Tony)Stewart]

Normally if TV accepts 30Hz from PC, it is interlaced or 60Hz non-interlaced... same for PAL

 

[amirke]

Ok,
Got it.

But in the other matter, the K70 can not handle with changing graphics of 800X600 LCD.
It is just to much information (30FPS) for the LCD controller to handle with.

 

[amirke]

I choose to work with ARM926 for this sort of LCD project.
Thanks